I had tried using OneIdentity, but this still seems to cause problems,
as the following session indicates.
In[1] := Clear[Int]
Attributes[Int] = {Flat, OneIdentity, Orderless, Listable}
?Int
Out[1] = Int
Attributes[Int] = {Flat, Listable, OneIdentity, Orderless}
In[2]:= Clear[x,y]
Int[x,y]
Out[2] = Int[x, y]
In[3]:= Int[x_] := x
In[4]:= Int[a]
Out{4}= a
In[4]:= Int[a,b]
General::recursion: Recursion depth of 256 exceeded.
Hold[Int[a,b]]
Out[4]= Hold[Int[a,b]]
Obviously it is trrying to do something very clever, and simplify a
function with two arguments, when the only rule it knows applies only in
the case of a single argument ... unfortunately, it doesn't seem to get
anywhere!